According to Tuesday's unofficial results, candidates from a coalition slate endorsed by Duterte held the top spots in the race for 12 of the Senate's 24 seats, with 94 per cent of election returns tallied.

One opposition candidate was trailing in 14th spot, while the rest were way behind.

Among the likely winners in the senatorial race were Duterte's former special assistant Christopher Go, who was in third place in the current tally, and his ex-police chief Ronald Dela Rosa, who was placing fifth.

Another ally in the winning circle is outgoing provincial governor Imee Marcos, daughter of late dictator Ferdinand.

Duterte said Monday's elections were a referendum on his rule, and a vote for his candidates would mean support for his policies, including a violent campaign against illegal drugs and a push for federalism.

The opposition is still hoping for one or two candidates to win seats in the Senate, considered as the last bastion of resistance to the 74-year-old leader's controversial policies.

"We will wait for the final results," said Senator Francis Pangilinan, campaign manager for the main opposition Liberal Party, noting that there were still three million votes uncounted in the unofficial tally.

"We will remain vigilant to the end."

Pangilinan added that whatever the results, the opposition would continue its struggle for justice, independence and prosperity against the Duterte administration.

Australian Associated Press